1. Field of the Invention
This invention pertains generally to excitation control systems for rotating electrical apparatus and, more particularly, to such systems which provide excitation voltage to a generator. The invention also pertains to methods for providing an excitation voltage for a rotating electrical apparatus.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in one or more bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle. Examples of such circuits are disclosed in U.S. Pat. Nos. 5,963,440; 5,963,441; 6,046,917; 6,208,120; and 6,232,751.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous generators and motors, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs).
A typical excitation control system includes a transformer, one or more controlled rectifier bridges, one or more bridge control modules and/or a firing control module for plural bridge control modules with a communication network between the modules, and a circuit breaker (e.g., a xe2x80x9c41 breakerxe2x80x9d) normally feeding AC power from the transformer to the controlled rectifier bridge(s). See Pat. Nos. 6,046,917; and 6,208,120.
One type of transformer is a three-phase power potential transformer (PPT) including three primary windings in a WYE-configuration and three corresponding secondary windings in a delta-configuration, although other transformer configurations may be employed (e.g., a delta-configuration in the primary and a delta-configuration in the secondary). See U.S. Pat. No. 6,232,751. The primary windings are interconnected with the AC phases from the generator, while the corresponding secondary windings are interconnected with the AC inputs of the bridge(s) by the circuit breaker. The PPT may be electrically interconnected with the terminals of the generator and, thus, that system is commonly referred to as a xe2x80x9cterminal fed excitation systemxe2x80x9d. Alternatively, the PPT may be electrically interconnected with any other suitable source of three-phase voltages. This alternative system is commonly referred to as an xe2x80x9cauxiliary bus fed excitation systemxe2x80x9d.
In a static excitation system, the PPT is electrically interconnected with a suitable (e.g., reliable) power source, such as, for example, the station power source or the generator terminals. When this power source is present and available, it may be employed to remove energy from the generator field relatively very quickly by a process known as xe2x80x9crapid de-excitationxe2x80x9d. Otherwise, when this power source is not available, energy is removed from the generator field at a relatively slower rate. xe2x80x9cRapid de-excitationxe2x80x9d is accomplished by phasing back the firing angle of the one or more rectifier bridges, in order that a suitable negative field excitation voltage is applied to the generator field.
As an alternative to employing a negative field excitation voltage to accomplish xe2x80x9crapid de-excitationxe2x80x9d, a similar result may be accomplished by electrically inserting (e.g., by employing a DC contactor or DC circuit breaker) a suitable discharge resistor in series with the field excitation voltage to the generator field.
When the generator is tripped, it is beneficial to remove the energy in the field excitation circuit as quickly as possible. A suitable way to accomplish this is through xe2x80x9crapid de-excitationxe2x80x9d. However, if the voltage of the PPT power source is too low, such as would happen with a high side fault (e.g., a relatively low impedance and, thus, a relatively low voltage short to ground) on the generator terminals, then the circuit breaker or xe2x80x9c41 breakerxe2x80x9d between the transformer and the controlled rectifier bridge(s) should be immediately tripped. Known excitation control systems handle this by providing two trip inputs: (1) emergency trip, which opens the circuit breaker or xe2x80x9c41 breakerxe2x80x9d; and (2) normal trip, which initiates xe2x80x9crapid de-excitationxe2x80x9d before the circuit breaker opens. Because it is not known whether rapid de-excitation can or cannot be successfully implemented, emergency trips result in an immediate opening of the xe2x80x9c41 breakerxe2x80x9d.
Accordingly, there is room for improvement in excitation control systems and methods.
These needs and others are met by the present invention, which provides improvements in an excitation control system for a rotating electrical apparatus. In accordance with the invention, the ability to rapidly de-excite determines what happens after a trip signal is input by the excitation control system. When the excitation control system receives the trip signal, it attempts xe2x80x9crapid de-excitationxe2x80x9d and, also, verifies that xe2x80x9crapid de-excitationxe2x80x9d is occurring. If xe2x80x9crapid de-excitationxe2x80x9d is not occurring, then the circuit breaker or xe2x80x9c41 breakerxe2x80x9d sourcing AC power to the controlled rectifier bridge is immediately opened.
The excitation control system initially attempts xe2x80x9crapid de-excitationxe2x80x9d in response to the trip signal. This may be accomplished by phasing back the firing angle of the rectifier bridge, in order to attempt to apply a negative field excitation voltage to the generator field. However, if a suitable power source voltage, such as a PPT voltage, is not present, then the system immediately opens the circuit breaker or xe2x80x9c41 breakerxe2x80x9d. Otherwise, if the suitable PPT voltage is present, then after a suitable time delay, the excitation control system verifies that the generator field voltage is sufficiently negative. If so, then xe2x80x9crapid de-excitationxe2x80x9d continues to be employed. Otherwise, if the generator field voltage is insufficiently negative, then xe2x80x9crapid de-excitationxe2x80x9d is not occurring and the system opens the circuit breaker or xe2x80x9c41 breakerxe2x80x9d.
In accordance with one aspect of the invention, an excitation control system for outputting an excitation voltage for a rotating electrical apparatus comprises: an electrical switching apparatus comprising a plurality of input terminals including a plurality of alternating current phases, a plurality of output terminals, a plurality of separable contacts electrically connected between the input and output terminals, and an input to open the separable contacts, the electrical switching apparatus providing the alternating current phases at the output terminals when the separable contacts are closed; a rectifier bridge comprising a plurality of inputs electrically interconnected with the output terminals of the electrical switching apparatus, a plurality of segments to convert the alternating current phases to the excitation voltage, and an output having the excitation voltage, each of the segments including an element having an input responsive to one of a plurality of firing signals; and a controller comprising: an input including a first signal, an output including a second signal, the output of the controller being electrically interconnected with the input of the electrical switching apparatus, a first value, a second value, a first sensor inputting at least one voltage corresponding to the alternating current phases of the electrical switching apparatus and outputting a sensed voltage, a second sensor inputting the excitation voltage of the output of the rectifier bridge and outputting a sensed excitation voltage, a plurality of outputs, each of the outputs electrically interconnected with the input of the element of a corresponding one of the segments of the rectifier bridge to provide a corresponding one of the firing signals thereto, a first function detecting the first signal and responsively controlling the firing signals of the outputs of the controller in order to provide the excitation voltage having a negative value, a second function, and a third function determining if the first signal is asserted and responsively delaying for a predetermined time, unless the second value is greater than the sensed excitation voltage, and after the predetermined time responsively asserting the second signal, in order to open the separable contacts of the electrical switching apparatus and remove the alternating current phases from the inputs of the rectifier bridge.
The first sensor of the controller may sense a plurality of voltages corresponding to the alternating current phases of the electrical switching apparatus and provide the sensed voltage as an average of the voltages corresponding to the alternating current phases.
The first sensor of the controller may sense a plurality of voltages corresponding to the alternating current phases of the electrical switching apparatus and provide the sensed voltage as a minimum of the voltages corresponding to the alternating current phases.
The third function may employ a timer having a first input to start the timer, a second input to hold or stop the timer, a predetermined time delay and an output, the first input of the timer including the first signal, the second input of the timer being asserted if the second value is greater than the sensed excitation voltage, the output of the timer being asserted the predetermined time delay after the first input of the timer is asserted, unless the second input of the timer is asserted. The second signal may be asserted when the output of the timer and the first signal are asserted.
The predetermined time delay of the timer may be about 20 mS to about 100 mS.
The output of the timer may be being asserted: (a) the predetermined time delay after the first input of the timer is asserted, unless the second input of the timer is asserted, or (b) when the sensed excitation voltage becomes greater than the second value.
As another aspect of the invention, an excitation control system for outputting an excitation voltage for a rotating electrical apparatus comprises: a transformer having a plurality of primary windings and a plurality of secondary windings, the primary windings being powered from alternating current phases of an output of the rotating electrical apparatus, the secondary windings having a plurality of alternating current phases; an electrical switching apparatus including a plurality of input terminals electrically connected with the secondary windings of the transformer, a plurality of output terminals, a plurality of separable contacts electrically connected between the input and output terminals, and an input to open the separable contacts, the electrical switching apparatus providing the alternating current phases of the secondary windings of the transformer at the output terminals when the separable contacts are closed; a rectifier bridge comprising a plurality of inputs electrically interconnected with the output terminals of the electrical switching apparatus, a plurality of segments to convert the alternating current phases to the excitation voltage, and an output having the excitation voltage, each of the segments including an element having an input responsive to one of a plurality of firing signals; and a controller comprising: an input including a first signal, an output including a second signal, the output of the controller being electrically interconnected with the input of the electrical switching apparatus, a first value, a second value, a first sensor inputting at least one voltage corresponding to the alternating current phases of the electrical switching apparatus and outputting a sensed voltage, a second sensor inputting the excitation voltage of the output of the rectifier bridge and outputting a sensed excitation voltage, a plurality of outputs, each of the outputs electrically interconnected with the input of the element of a corresponding one of the segments of the rectifier bridge to provide a corresponding one of the firing signals thereto, a first function detecting the first signal and responsively controlling the firing signals of the outputs of the controller in order to provide the excitation voltage having a negative value, a second function determining if the first value is greater than the sensed voltage of the first sensor and if the first signal is asserted, and responsively asserting the second signal in order to open the separable contacts of the electrical switching apparatus and remove the alternating current phases from the inputs of the rectifier bridge, and a third function determining if the first signal is asserted and responsively delaying for a predetermined time, unless the second value is greater than the sensed excitation voltage, and after the predetermined time responsively asserting the second signal, in order to open the separable contacts of the electrical switching apparatus and remove the alternating current phases from the inputs of the rectifier bridge.
The output of the rotating electrical apparatus may include three alternating current phases, and the primary of the transformer may include three primary windings in a WYE-configuration for the three alternating current phases.
The primary of the transformer may include three primary windings for three alternating current phases, and the secondary of the transformer may include three secondary windings in a delta configuration for the three primary windings.
As another aspect of the invention, an excitation control system for outputting an excitation voltage for a rotating electrical apparatus comprises: means for sourcing a plurality of alternating current phases; means for switching the alternating current phases of the means for sourcing in response to an input; means for converting the switched alternating current phases from the means for switching to the excitation voltage responsive to a plurality of firing signals; and a controller comprising: an input including a first signal, an output including a second signal, the output of the controller being electrically interconnected with the input of the means for switching, a first value, a second value, first means for inputting at least one voltage corresponding to the alternating current phases of the means for sourcing and outputting a sensed voltage, second means for inputting the excitation voltage and outputting a sensed excitation voltage, means for outputting the firing signals, means for detecting the first signal and responsively controlling the firing signals in order to provide the excitation voltage having a negative value, means for determining if the first value is greater than the sensed voltage of the first means for inputting and if the first signal is asserted, and responsively asserting the second signal in order to remove the switched alternating current phases from the means for converting, and means for determining if the first signal is asserted and responsively delaying for a predetermined time, unless the second value is greater than the sensed excitation voltage, and after the predetermined time responsively asserting the second signal, in order to remove the switched alternating current phases from the means for converting.
As another aspect of the invention, a method for providing an excitation voltage for a rotating electrical apparatus comprises: sourcing a plurality of alternating current phases; switching the alternating current phases in response to an input; converting the switched alternating current phases to the excitation voltage responsive to a plurality of firing signals; inputting a first signal; outputting a second signal as the input; employing a first value; inputting the excitation voltage and outputting a sensed excitation voltage; detecting the first signal and responsively controlling the firing signals in order to provide the excitation voltage having a negative value; and determining if the first signal is asserted and responsively delaying for a predetermined time, unless the first value is greater than the sensed excitation voltage, and after the predetermined time responsively asserting the second signal, in order to remove the switched alternating current phases.
The method may further comprise employing a second value; inputting at least one voltage corresponding to the alternating current phases and outputting a sensed voltage; and determining if the second value is greater than the sensed voltage and if the first signal is asserted, and responsively asserting the second signal in order to remove the switched alternating current phases.